The invention relates to a method of depositing microcrystalline solid particles from the gas phase by means of Chemical Vapour Deposition (CVD), in which the solid particles are deposited on a substrate heated to a temperature ranging between 450.degree. and 1200.degree. C. at a pressure ranging between 10.sup.-5 and 1 bar and in a directed gas flow.
The deposition of microcrystalline solid bodies from the gas phase by means of Chemical Vapor Deposition (CVD) is widely used for manufacturing self-supporting moulded bodies or for forming layers on substrates of different types.
A general characteristic feature of a CVD process is that a mixture of gases interacts at a relatively high temperature interacts while decomposing at least one gas in the gas mixture and depositing a reaction product in a fixed phase on a substrate surface. The conventional CVD process requires high temperatures, reactive gases and a device which is suitable for performing such a method. Typical process parameters are temperatures ranging between 450.degree. and 2500.degree. C., pressures ranging between 10.sup.-5 and 1 bar and a reactant gas mixture comprising at least one reactive gas and additional gases such as inert, oxidizing or reducing gases.
Accordingly, the reaction products (solid particles) are obtained by
pyrolysis (for example: CH.sub.3 SiCl.sub.3 .fwdarw.SiC+3HCl) or PA0 reduction (for example: WF.sub.6 +3H.sub.2 .fwdarw.W+6HF) or PA0 oxidation (for example: SiH.sub.4 +O.sub.2 .fwdarw.SiO.sub.2 +2H.sub.2) or PA0 co-reduction (for example: TiCl.sub.4 +2BCl.sub.3 +5H.sub.2 .fwdarw.TiB.sub.2 +10 HCl).
The reactant gas mixture is activated within the reactor in an excitation zone (gas phase range with maximum energy content), which can be generated through an inductively heated wire or a flame via a plasma which is obtained by coupling in microwave energy or high-frequency energy or energy from an electric d.c. field. In the excitation zone the gas phase reactions are stimulated and the reaction products are subsequently deposited on the substrate which is preferably positioned at a distance from the excitation zone on a conventional temperature-controlled substrate holder. The residual gases are exhausted from the reactor by means of a vacuum pump.
Energetic excitation via resistance heating of a gas phase for a CVD process for depositing diamond in a tubular reactor is known from the publication by W. Yarbrough and R. Roy in Extended Abstracts of the Spring Meeting Session of the Materials Research Society, USA, April 1988, pages 33 etc. This state of the art method is shown diagrammatically in FIGS. 1a and 1b. The reactant gas mixture 3 is introduced into the tubular reactor 1 and brought to the required reaction energy level within an excitation zone (zone having a maximum energy content) which is generated by the resistance heating member 4.
The activated reactant gas mixture 33 exits at the end of the reactor 1 and diamond is deposited on a substrate 9 arranged at a distance from the reactor 1. FIG. 1b diagrammatically shows the excitation zone as a section with its temperature profile T throughout the cross-section of the reactor 1. The drawback of such a process is the inhomogeneity of the excitation throughout the cross-section of the gas phase to be excited. Under circumstances a part of the gases, for example, in the center of the reactor tube may not yet be energetically brought to the reaction level, while other parts, for example, at the reactor wall are already excited to such an extent that the deposition reactions already lead to a homogeneous nucleation of the materials to be deposited in the gas phase instead of to a formation of layers on the substrate. In addition to the formation of particles already in the gas phase there is the further unwanted effect that the layer deposited on the substrate is inhomogeneous as far as composition and layer thickness are concerned.